Method of reducing plutonium with ferrous ions



i and United States Patent METHOD OF REDUCING PLUTONIUM WITH FERROUS IONS No Drawing. Application December 2, 1947 Serial No. 789,332

=11 Claims. 01.23-14. 5 V

invention relates to the separation of plutonium from elements commonly found with it, and more particularly isconcernedwith an improvement in the oxida tion-reduction method'of separating plutonium from other elements. 'Plutonium is the elementwith atomicnumber 94. Reference herein to plutonium or to other elements to be understood as denoting the element generically whether free state or in the form of a compound, unless otherwise indicated by the context.

Plutonium is ordinarily obtained by irradiating uranium with neutrons, It is well known that the plutonium thus obtained is,,a small part of the irradiated uranium mass and that other-constituents. of the mass include, as radioactive fission products, elements of the group having atomic numbers between 32 and 63. Many of these radioactivefission products have great value as tracers invarious fields of research particularly in the physical, medical, and chemical'fields and it is therefore highly desirable t-hat thefplutonium be separated from these fission products and from the uranium with which it is associated. 1 The. mostsuccessful method of separating plutonium from'fthe other constituents of an irradiated uranium mass is the oxidation reduction methodfwhich depends upon the fact, that plutonium exists in more than onestate of oxidation, and that compounds formed by plutonium in these various oxidation states diifer widely in their solu- Hility characteristics. e 1 e The inost important of the oxidation-reduction separation proTcesses is the bismuth; phosphate-lanthanum fluoride process. This. process may be arbitrarily divided into four sequences of operations o'r'steps. These are usually referred to asthe extraction, decontamination, concentration'and isolation steps. Many of the operations-in these steps are cyclic in nature and these operation's may-be repeated as often as necessary to achieve the desiredeffect. In the description which follows, a precipitate containing plutonium will he-referredto as a productprecipitate, and a precipitate which carries other elements from a solution containing plutonium in avalence state in which it is not carried with the particular precipitate, is referred to as a by-product precipitate)? In, the extraction step of the bismuth phosphate-lanthanurriifluoride process the neutron-reacted uranium is aged from to ninety days to permit the neptunium present ii -the uranium to decay. to plutonium. The uranium massis then dissolvedin nitric acid to form a uranyl-"nitrate solution containing all of the uranium as well as the plutoniurn and fission products. Dissolution of the uranium mass in 'thehot nitric acid l causes some oxidation: of the .plufonium present to its=highest oxidation state,.t he -|-.6 state,.in which state plutonium forms a soluble phosphate; The solution. is therefore contactedwith a reducirifgjagent and the plutonium con: verted to the tetravalent state in which state it formsan insoluble phosphate. A' plutonium carrier precipitate of bismuth phosphate is then formed in the uranyl nitrate solution and separated from the solution carrying with it the tetravalent plutonium and certain of the fission products, which form insoluble phosphates, particularly zirconium and columbium, leaving the uranium and bulk of the fission products in solution. This separation of the product precipitate completes the extraction step.

The operationswhich follow the extraction step are designed to separate the plutonium from the radioactive fission products which are carried with it in the first plutonium precipitation and are usually referred to asthe decontamination step. In this step the bismuthphosphate-plutonium carrier precipitate obtained in the extraction step is dissolved in nitric acid and the plutonium is oxidized to the hexavalent state. A bismuth phosphateprecipitateis then formed in and separated from the solution containing the plutonium and fission products. 'The phosphate-soluble hexavalen't' plutonium remains in solution during this by-product precipitation but the phosphate-insoluble fission products are carried with the bismuth phosphate precipitate. The plutonium is then reduced to the tetravalent state and separated from the solution by forming a bismuth phosphate carrier precipitate in the solution and separating it therefrom. This operation completes the decontamination step but this step may be repeated as many times as necessary in order to secure the decontamination desired.

The bismuth phosphate product precipitate obtained in the last operation of the decontamination step contains a very small proportion of plutonium to' bismuth phosphate and it is desirable that the ratio of plutonium to carrier be increased. This is accomplished by the concentration step which follows. In this step the hismuth phosphate product precipitate is dissolved in nitric acid and the plutonium is oxidized to the hexavalent state. then substantially removed by precipitation and the plutonium is converted to the tetravalent state. A lanthanum fluoride-plutonium carrier precipitate is formed in, and separated from, the solution. The lanthanum fluorideplutonium carrier precipitate is dissolved in nitric acid may have been oxidized to the hexavalent state by the warm nitric acid. The tetravalent plutonium is then precipitated from solution as the peroxide to complete the isolation step and the separation process.

There are several alternate procedures involving the use of carriers other than bismuth phosphate and lanthanum fluoride and containing otherv minor variations in the above-described process. The oxidation-reduction process, in generah'however, follows the outline above.

It will be noted that each step of the above-described bismuth phosphate-lanthanum fluoride plutonium separation process contains at least-onep'lutonium reduction operation. In each of these operations the plutonium is converted in an acid'medium fromthe hexavalent to the tetravalent state, in which state it is carried quantitatively by either bismuth phosphate or lanthanum fluoride Because of the small percentage of plutoniuminvolved in the process; it is essential that the utmost efliciericy be The bismuth phosphate present in the solution is This mixed hydroxide is separated from the of research has been done upon this problem. Numerous.

reducing agents have been used including S N H NH OH, and U. All of these have been found, however, to have certain disadvantages. For example, some of the agents reduce the plutonium too slowly or require such high temperatures in order for the reduction reaction to proceed that there is excessive corrosion of the process vessels by the acid solution. Other agents have such positive oxidation-reduction potentials that they reduce the plutonium to the trivalent state in which state it carries poorly with most carrier precipitates. It is often diflicult to remove the excess reducing agent, and after reduction of plutonium to the trivalent state, this excess reducing agent tends to prevent the subsequent oxidation of the plutonium to the quadrivalent state. Other reducing agents introduce undesirable ions into the solution, such as the colloidal sulfur often produced when S0 is used. r

It is an object of this invention to provide a method for reducing plutonium from a higher to a lower valence state.

'It is an additional object of this invention to provide an improvement in the plutonium reduction steps of the bismuth phosphate-lanthanum fluoride plutonium separation process.

Added objects and advantages of the present invention will be evident from the following description.

In accordance with the present invention, it has been found that the ferrous ion is a most effective reducing agent for plutonium. The oxidation-reduction potential of the ferrous-ferric couple is sufiiciently less' negative than the potentials of the plutonium couples that ferrous ion will readily reduce plutonium from any higher to any lower valence state. The oxidation-reduction potentials of the various plutonium couples and of the ferrousferric couple are as follows:

The method of reducing either the hexavalent or tetravalent plutonium ion to the trivalent state comprises treating the oxidized plutonium ion contained in an aqueous acidic solution with ferrous ion.

The quantity of ferrous ion required to reduce plutonium may be readily calculated from the equations given above. It has been found desirable to use amounts of ferrous ion somewhat larger than stoichiometric proportions in order to increase the rate of reduction, unless the reduction is carried out in the presence of other ions which would give undesirable side results with excess ferrous or ferric ion.

Any soluble ferrous salt, such as the ferrous chloride, nitrate, or sulfate, is'a suitable agent for introducing the ferrous ion into the solution. Ferrous ammonium sulfate has been found to be a very satisfactory reagent for this purpose. It is a crystalline salt readily soluble in water and'can be dissolved to form an aqueous solution which is easily handled. The preferred Fe(NH (SO .6H O solution is a 20% solution containing 0.1% sulfuric acid; the sulfuric acid tends to stabilize the ferrous salt by preventing hydrolysis. The reduction reaction of plutonium ions with ferrous ion is carried out in an acid solution. The reason for this is that the hexavalent, tetravalent and trivalent plutonium ions all form insoluble hydroxides in basic solutions. There is also a strong tendency for the PU ion to disproportionate to the +3 and +6 states at low acid concentration. For these reasons the reduction is best carried out in a moderately acid solu tion, for example, 0.5 to 1.5 N HNO The time and temperature which thereaction requires are quite dependent on each other. Thus, at temperatures from 5090 C. the reduction takes place almost '4 instantaneously, however, at room temperature it may take a much longer time.

The PM ion is difficult to convert to the quadrivalent state directly, since the reduction potentials of the various plutonium couples are so close together that a reducing agent which will cause the reduction of hexavalent plutonium to quadrivalent plutonium will also normally cause the reduction of the hexavalent'plutonium to the trivalent state. The conversion of hexavalent plutonium to the quadrivalent state may, however, be brought about by reducing the hexavalent plutonium to the trivalent state and then oxidizing the trivalent plutonium to the quadrivalent state.

We have discovered that where hexavalent plutonium is reduced with ferrous ion in a nitric acid solution, the excess ferrous ion may be oxidized to the ferric state and the trivalent plutonium ion may then be oxidized to the quadrivalent state by digesting the nitric. acid solution containing the plutonium at an elevated temperature for a suitable time. The ferrous ion isfirst completely converted to the ferric state followed almost instantaneously by the conversion of the trivalent plutonium to the quadrivalent state. This conversion can often be observed by a color change which usually occurs in the nitric acid solution. Agreenish-brown color indicates the presence of the ferrous ion and this color intensifies as the potential rises but disappears at the final conversion of the ferrous to the ferric state. It is presumably caused by a ferrous-nitroso complex. The time required for the conversion of the trivalent plutonium to the'quadrivalent state is somewhat dependent upon the temperature of the reaction solution, and the acid concentration. 'Where the solution was made .04 M in ferrous ion, ferrous ion was completely oxidized by 0.5 N nitric acid in .five minutes at 75 C. and by 0.3 N nitricacid in ten minutes at the same temperature. However, at 50 C. an 0.04 Fe* concentration required three hours in 0.3 N nitric acid, twenty-seven minutes in 0.5 N nitric acid, and less than five minutes in 1 N nitric acid for complete conversion to the ferric state.

The effect of the temperature of digestion upon the length of time required to convert the ferrous to the ferric ion, was shown by experiments in which it was found that ferrous ion may last as .longas sixty-sixminutes at 25 C. in a l.0 N nitric acid solution. The solution condi-' tions of the experiment simulated those of the solution containing hexavalent plutonium which is converted to the tetravalent state in the decontamination step of the bismuth phosphatelanthanum fluoride plutonium separation process. At 50 'to 75 C., the ferrous ion is completely oxidized to the ferric ion within five minutes under similar conditions. Since it has also been shown that trivalent plutonium is oxidized to the tetravalent state in the above-mentioned solution almost immediately following the oxidation of the last of the ferrous ion present, these time factors may be considered to apply to the time required to convert the trivalent to the tetravalent plutonuim under these conditions. 7

An alternate method of converting hexavalent to quadrivalent plutonium depends upon the effect of quadrivalent-plutonium-complexing agents contained in the reaction solution. Certain ions, particularly fluoride, oxalate, and phosphate ions, strongly complex plutonium in the tetravalent state. The effect of the presence of a complexing agent on the oxidation-reduction potential may be shown by the following equation in which hydrochloric acid was substituted for nitric acid in order to prevent the slow drift of potentialcaused by nitric acid oxidation of trivalent plutonium.

In the fiuoride solution ferrous ion will reduce hexavalent plutonium to the tetravalent state but ferrous ion will not reduce plutonium to the trivalent state. More: over, the, lu (IV)/iu (VI) potential is shifted in the direction of making Pu (VI) a more powerful oxidizing agent in the presence of the fluoride ion so that less ferrous ion is required to reduce Pu (VI) to Pu (IV) than would be required for the same reduction in the absence of fluoride ion.

In the bismuth phosphate-lanthanum fluoride process, which has been previously described, there is a plutonium reduction operation in each of the four steps: the extraction, decontamination, concentration, and isolation step. The plutonium in each case is converted from the hexavalent to the tetravalent state. Ferrous ion may be used in the process of this invention in the reduction operation in each of these steps and has proved to be particularly advantageous inthe decontamination step. In this step plutonium is converted from the hexavalent to the tetravalent state following a by-product precipitato maintain the plutonium in the hexavalent state during the bismuth phosphate by-product precipitation. Following the conversion of the hexavalent plutonium to the tetrav'alent state in this solution a bismuth phosphate product carrier precipitate is formed in the solution and separated therefrom, thus separating the plutonium from foreign cations contained in the solution which are not carried withbismuth phosphate.

The apparent disadvantages ofthe use of ferrous ion as a reducing agent in the decontamination cycle of the bismuth phosphate-lanthanum fluoride process, namely, that ferrous ion forms ast'roh gl'y bb'iind' complex with the phosphate ion, and that ferrous ion reduces hexavale'nt plutonium to "he trivalent state, are overcome by the proe'ess of thi' venues, The disadvantage bf'the formation of ferrons ph6sphate complex ion is overcome by using only a small excess of ferrous ion over that required to reduce the he'X'a'Valent plutonium 'and any oxidizing agent present and by increasing the phosphate ion concentration in the solution to about L6 M H PO The reduction of the plutonium'to the ditficultly carriable trivalent state is overcome as described above either by oxidizing the trivalent plutonium forme d to the tetravalent state by digestion "at moderatelyelevated temperature in nitric acid solution, or by reducing the plutonium in the presence of -a complexiirg agent forquadrivalent plutonium. 1

There are characteristics of the ferrous sion which greatly, increase the decontamination of plutonium in this stage-.- For example, ferrous and ferric ions tend to solubilize rare earth phosphates which are presentin solution;- thus lanthanum phosphate present in solution is carried by a bismuth phosphate precipitate to the extent 'of 30% in the absence of ferrous ion but of' only 3% in its presence. This tends to prevent the rare earth fission products in solutionfrom carrying with the product bisrnuth phosphate carrier precipitate following a ferrous reduction step.

The improvement in the decontamination cycle- 0btained when'ferro'us' ion isused as a reducing agent by the process of this invention is shown by the following table in which the decontamination results obtained'when ferrous ionis used arefcomparedwith those. obtained with a uranous ion reducing agent and an oxalate reducing agent. It will benoted that the bismuth phosphate product precipitate carries at least twice as much of the radioactive fission products (which are 5- and 'y-emitters) after a uranous reduction and four times as much after 6 an oxalate reduction, as after the ferrous reduction; "me conditions of the experiments shown in the examples were as follows: three aliquots of a solution containingl N HNO 0.1 M H PO 0.002 M Kzcl'zoq, a tracer quantity of hexavalent plutonium and tracer quantities of fission product elements were reduced with ferrous ion by the process of this invention, uranous ion and oxalate ion, respectively. A bismuth phosphate product precipitate was then formed in and separated from each of the three aliquots and the precipitate and supernatant solutions were then analyzed for plutonium and for fission products by radiometric analysis of alpha, beta, and gamma counts. The presence of plutonium is indicated by percentage of alpha counts, and the presence of fission products by the percentage of beta and gamma counts. 7 7

TABLE I Percent of Activity Oxidized Solution Method of Reduction Fraction Analyzed Found in Fraction Analyzed a B 'y BiPOl produotindecont'am- 0.03M Fe+ (1 hr. at ination cycle 85 2.5 4 Waste supernatant after 1 product pptn 18 94 101 BifiPO ptroduct 1in decon 97 9 8, amma 1011 0Y0 e M U Waste supernatant after product pptn 18 '86 BitP0 yroduct 1in decon- 71 24. 18 v amma 1011 'cyc e M05 M H1020 Waste supernatant after product pptn '24 63 '84 about-0.03 M in ferrous ion in excess of the holdingoxidant present, thentdigestin-g the solution at 75, C.

with agitation for one hour. The plutonium which is present in about 1x10- M concentration is almost immediately reduced to the trivalent state and theone-hour digestion 'at elevated temperature which follows completely oxidizes the excess ferrous ion to the ferric state and the trivalent plutonium to the tet'ravalent state. The tetravalent plutonium may then be removed from the solution by formation and separation of a bismuth phosphate carrier precipitate. i

Variables which affect the reduction'step include the ferrous concentration, temperature, order of addition, 'ni tric 'acid concentration, and method of forming the subsequent bismuth phosphate carrier precipitate. Under normal conditions, a ferrous concentration of between 0.002 M and 0.05 M ferrous ion in excess 'ofthe holding oxidant present has been found to give-entirely satisfactory reduction. At lower concentrations the iron may be oxidized by air and nitric acid before reducing allo'f nitric acid concentration and :75 fC.,'wehave foundfthat "a one-hour digestion period is ample to oxidize any exf cess ferrous ion to ferric ionandall"trivalentplutonium tothe tetravalent state. .Under normal; conditions, [the temperatureandtime could be'consider'ably reduced'without adversely "affeeting the conversion ofthe plutonium and subsequentv carrying of the tetravalent plutonium by the bismuth phosphate precipitate.

yThe preferable manner of forming the bismuth-phosphate-tetravalent-plutonium carrier following the reduction step is the direct strike method, i.e., the bismuth ion is added to the solution prior to the addition of the phosphate ion. Should the reverse strike, which is the addition of the phosphate ion prior to the addition of the bismuth ion, be used, however, it is desirable to use a smaller quantity of ferrous ion in the preceding reduction. Thus, tests have shown that a ferrous ion concentration of, 0.03 M will cause consistent losses of when a reverse strike is used. At a 0.01 M ferrous concentration, however, no losses will occur'because of the ferrous concentration when the bismuth phosphate precipitate is formed by either method.

Now that the plutonium reduction has been described, its use in the decontamination cycle of the bismuth phosphate-lanthanum fluoride plutonium separation process may be illustrated by the following example.

Example The steps of the bismuth phosphate-lanthanum fluoride process, as described above, have been carried out through'the first by-product precipitation of the decontamination cycle. One and sixty-four/hundrdedths lb. of Pu(NO is contained in 20,400 lbs. of the solution. The components of the solution include 1680 lbs. of nitric acid, 183 lbs. of phosphoric acid, 8 lbs. of sodium dichromate, 1 lb. of ammonium nitrate, 4 lbs. of sodium nitrate, and 18522 lbs. of water. To this solution 1575 lbs. of 14% ammonium silicofluoride solution is added, and then 1950 lbs. of 20% FeSO (NH SO .6I-I O solution is added. The solution is heated to 75 C. and maintained at this temperature for one hour with constant agitation. .Two hundred sixteen lbs. of anaqueous bismuth solution containing 24% BiONO and 19% HNO is then added to the'solution at the rate of 3 /2 -lbs. per minute and the solution is agitated for thirty minutes at 75 C. Nineteen hundred forty-five lbs. of a phosphoric acid solution containing 73.5% H PO and 1.2% HNO is'added to the solution and the solution is agitatedfor two hours at 75 C. The solution is then cooled to 50 C. and the bismuth phosphate plutonium carrier precipitate is separated by centrifugation Another modification of the invention is concerned with the conversion. of plutonium from the hexavalent to the tetravalent state, said' plutonium being contained in a solution with a holding oxidantof dichromate ion present, It has been found that hydrogen peroxide will reduce dichromate ion, but not hexavalent plutonium, and this modification comprises treating the solution containing the hexavalent plutonium and dichromate ion, with a suflicient. quantity of hydrogen peroxide to reduce the dichromate ion, and then introducing a sufiicient amount of ferrous ion to reduce the hexavalent plutonium to the tetravalent state.

This modification has been foundto be very useful in the conversion'of hexavalent plutonium to the tetravalent state in the concentration cycle of the bismuth phosphatelanthanum fluoride plutonium separation process. @In this cycle, the plutonium is converted to the tetravalent state and then separated from solution with a lanthanum fluoride carrier precipitate. The carrying of plutonium by lanthanum fluoride is inhibited with a smaller .quantity'of iron than is the case when tetravalent plutonium is carried with a bismuth phosphate precipitate. This modification is, therefore, particularly useful in this concentration cycle, since the conversion of the plutonium to the tetravalent state is elfected'with a much smaller quantity of ferrous ion than would be required if both the dich'romate and the hexavalent plutonium were reduced by ferrousion alone.

tAnother situation in which this modification has proved particularly useful, is where hexavalent plutonium is converted'to the tetravalent state with fluoride ion present in the reduction solution to complex the tetravalent plutonium formed and prevent its further reduction to the trivalent state. The presence of the fluoride ion would cause undesirable corrosion in the reaction vessels if the reaction solution were heated much above room temperature, but if the reaction is carried out at room temperature, either a larger amount of ferrous ion or a much longer time will be required to eifect thereduction of the plutonium with ferrous ion alone for the reduction of the dichromate ion and the hexavalent plutonium. This modification is particularly desirable in this situation then, because the dichromate ion may be reduced at room temperature with hydrogen peroxide and a small quantity of ferrous ion will besufficient to efiect the reduction at room temperature of the hexavalent plutonium.

The hydrogen peroxide is added in an amount approximately equivalent to the dichromate ion contained in solution. Between and 115% ofthe stoichiometrically equivalent amount has been found satisfactory. The dichromate is almost instantly reduced and a small amount of ferrous ion is then added to reduce any remaining peroxide and to reduce the plutonium. Where the plutonium is present in solution in approximately .0001 M concentration, an iron concentration of 0.015 to 0.20 M (22-33% of the amount equivalent to the initial dichromate concentration) has been found to give satisfactory results, where the solution is 1.0 N in HNO and 1.0 N in HP. The optimum conditions for the reduction of plutonium where it is present in a solution in approximately 0.0001 M concentration, the solution also containing 1.0 N HNO 1N HF, and 0.1 N K Cr O are: addition of H 0 in amount suflicient to make the solution 0.075 M, agitation for one-half hour at room temperature, followed by the introduction of a suflicient quantity ,of ferrous ion to make the solution 0.02 M in ferrous ion, and agitation for'an additional one-half hour at room temperature. p

The H 0 concentration may be decreased somewhat by increasing the Fe+ concentration, but the H 0 con centration should not be increased greatly over 0.075 M, since any excess would tend to oxidize the ferrous ion added later, thus preventing complete reduction of the plutonium.

Now that the results of the variation of factors in this modification have been discussed in general, the eflect of .variations may be illustrated by the following tables in which particular conditions are shown. These tables are given by way of illustration and not to limit the invention. Unless otherwise specified the conditions of the experiments below were as follows: 0.001 M Pu/ liter, 1 N HF, l N HNO 0.1 N Kzcl'zoq. The solutions were prepared by oxidizing plutonium with 0.1 N K Cr O in 1 N HNO solution after which the solutions were made 1 N in HF. The hydrogen peroxide and ferrous ion were added according to the conditions listed below. Then a lanthanum nitrate salt was added to cause TABLE Ill REDUCTION WITH 0.02 M Fe++ AT VARYING Hi0: CONCEN- 'IRATIONS AND TEMPERATURES [Conditionsal N HF, 1 N HNOa, 0.1 N KzClzO Fe added 36 hour after add tion of H202, Fe in solution 1 hour including hour period of precipitation of preformed LaFa, temperature at; indicated leve] throughout] H202 Fe Temp. Percent (moles/1.) (moles/l.) O.) Pu Reduced v M TABLE I11 REDUCTION AT ROOM 'rgixgrnnlt'ruan UsriIo 0.05 M

[Conditions Same as Table 11.]

REDUCTION AT ROOM TfihgPERATURE USING 0.075 M [Cnditions: Same as Table 11.]

H202 -Fe++ Temp. Percent (moles/l.) ,(moles/l.) C.) Pu

7 Reduced TABLE V REDUCTION AT ROOM 'fiEWPERATURE USING 0.1 M

[Conditions: Same as TableIL] H202 Fir l( enc.u3. Pei cent moles mo es u I1) [1 Reduced the formation of 'a lanthanum fluoride precipitate, and this was digested for one-half hour before the preclpitate was separated by centrifuging. The amount of reduced plutonium which was carried by the lanthanum fluoride precipitate was determined by radiometric analysis.

While there have been described certain embodlments of our invention, it is to be understood that it is capable of many modifications. For example, it may beusedv not only in. all of the reduction steps of the bismuth phosphate process, but also in the reduction steps of many other oxidation-reduction plutonium separation processes. Changes, therefore, may be made without departing from the spirit and scope of the invention as described in the appended claims, in which it is the intention to claim all novelty and invention as broadly as possible in view of the prior art.

What is claimed is: p 1. The method of reducing plutonium from the hexavalent to the tetravalent state, which comprises contacting said hexavalent plutonium'contained in an aqueous acidic solution, with ferrous ion in the presence of a complexing agent for tetravalent. plutonium selected th trivalent pl tonium conv rted t the tetraval nt state. I 4. The method of separating dissolved hexavalent plutonium from an aqueous nitric acid solution, which comprises treating said solution with an excess of ferrous ion over the amount required to reduce the hexavalent plu-' tonium to the trivalent state, then digesting the solution at a temperature of from 25 to 100 C. until the excess ferrous ion is converted to the ferric state and the trivalent plutonium converted to the tetravalent state, then forming a bismuth phosphate precipitate in s'aidsolution and separating it therefrom together with the associated plutonium. 7 I

5. The method of separating dissolved hexavalent plutonium contained in an aqueous 0.5-1.5 N NHO solution also containing a holding oxidant of CI'207= ion, which comprises treating said solution with an amount of ferrous ion in excess of that necessary to reduce substantially completely the dichromate ion present and to reduce the hexavalent plutonium present to the trivalent state, then digesting the solution at between 50 and 100C. until the ferrous ion present is converted to the ferric state and the trivalent plutonium converted to the tetravalent state, then forming a bismuth phosphate precipitate in the solution and separating it, together with associated tetravalent plutonium, therefrom.

6. The method of separating hexavalent plutonium from an aqueous solution containing 0.5-1.5 N NHO approximately 0.6 M H PO and approximately 0.1 M K Cr O which comprises introducing ferrous ion in sufficient quantity to give a ferrous concentration of between 0.0025 and .03 M after reduction of the dichromate ion present, digesting the solution at a temperature between 40 and 90C. until the excess ferrous ion is oxidized and the trivalent plutonium is oxidized to the tetravalent state, then forming a bismuth'phosphate precipitate in the solution by introducing a soluble bismuth compound and a soluble phosphate compound in that order, and separating the bismuth phosphate precipitate thus formed together with associated plutonium from the solution. I

7. The method of separating hexavalent plutonium from an aqueous nitric acid solution containing hexavalent plutonium and dichromate ion, which comprises introducing into the solution an amount of hydrogen peroxide sufficientto reduce substantially completely the dichromate ion present, then introducing ferrous ion in an amount sufllcient to reduce the hexavalent plutonium ion present 'to the tetravalentstate, then forming a tetravalent plutonium carrier precipitate in the-solution and separating it therefrom.

-8. The method of separating hexavalent plutonium from aqueous I-INO solution also containing dichromate ion and fluoride ion, which comprises introducing hydrogen peroxide suflicient to reduce substantially completely the dichromate ion present and then introducing ferrous ion in excess of the theoretical amount necessary to reduce the hexavalent plutonium ion present to the tetrafrom the group consisting of fluoride, oxalate and phosphate.

. l 2. The method of reducing plutonium from the hexavert the plutonium to the trivalent state,'then digesting the solution at a temperature between 25 and 100 C. until the ferrous ion is converted to the ferric state and valent state, then forming a carrier precipitate for tetravalent plutonium in the solution and separating the precipitate therefrom.

9. The method of separating plutonium from an aqueous nitric acid solution containing hexavalent plutonium,

rare earth metal values contained in an aqueous nitricacid solution and containing the plutonium in the hex-avalent state, comprising adding ferrous ions to saidsolution and heating said solution to 50 to C. whereby said rare earth metal values, primarily due to the solubil- 5 izing efiect of iron ions, remain in solution.

11. A method of separating: plutonium values from rare earth metal values contained in an aqueous nitric acid solution and containing the plutonium in the hexavalent state, comprising adding ferrous ions and fluoride anions to said solution whereby the plutonium is secured in the tetravalent state, thereafter adding bismuth cations and phosphate anions to said solution whereby bismuth phosphate and plutonium (IV) phosphate are precipi- 12 tated while said rare earth metal values, primarily due to the solubilizing effect of iron ions, remain in solution.

References Cited in thefile of this patent Handbook of Chemistry and Physics, 26th ed., p. 1337 (1942). Pub. by Chemical Rubber Publ. Co., Cleveland, Ohio.

Connick et al.: Note on the Oxidation of Pu by Nitrate Oflice of Technical Services, PB52753, MDDC Report 337, July 1946.

Harvey et al.: Journal of the Chemical Society, August 1947, pages 1010-1021. 

1. THE METHOD OF REDUCING PLUTONIUM FROM THE HEXAVALENT TO THE TETRAVALENT STATE, WHICH COMPRISES CONTACTING SAID HEXAVALENT PLUTONIUM CONTAINED IN AN AQUENOUS ACIDIC SOLUTION, WITH FERROUS ION IN THE PRESENCE OF A COMPLEXING AGENT FOR TETRAVALENT PLUTONIUM SELECTED FROM THE GROUP CONSISTING OF FLUORIDE, OXALATE AND PHOSPHATE. 